When a paste is mixed from two or more components, specifically a liquid and a powdery component for producing a bone cement, it is essential for the mixture to be made as homogeneous as possible and to be completely free of bubbles to obtain a high strength after setting. To this end, any air included during the mixing process must be subsequently removed. If the product is a bone cement having a typical setting time of several minutes, only short time is available for preparing the mixture and removing the air.
German Offenlegungsschrift 3,708,442 discloses a mixing apparatus for pastes which comprises a rotor rotatable about a first axis and including a mount for receiving a mixing container which holds the components of the paste to be prepared, the mount being supported on the rotor for rotation about a second axis eccentric with respect to the first axis, and a transmission for causing the mount to rotate about the second axis while the rotor rotates about the first axis.
In the known device, the components to be mixed are contained in a common container and are separated in a transport condition of the container by a partitioning wall. In use, the partitioning wall is first of all ruptured, and the container is inserted into the mount of the mixing apparatus. Resilient means retain the mount relative to a base of the apparatus in such a manner that, while the rotor rotates, the general orientation of the mount is maintained, so that the container accommodated in the mount is subjected to a circular reciprocating motion. Upon termination of the mixing process, the container is connected to a vacuum pump to degas the mixture. Thereafter, the container is connected to a dispensing device in which a plunger acts on a piston located at one end of the container to dispense the finished mixture through a dispensing opening provided at the other end of the container.
Similar mixing apparatus are described in U.S. Pat. No. 4,131,369 and East German Patent 94,554. In the former apparatus, the mixing container is mounted on a planet gear which meshes with the sun gear and the outer ring gear of an epicyclic gearing. By periodically changing the relative angular position between the sun and ring gears, the container is intermittently rotated back and forth by 180.degree..
Since the mixed paste has a highly viscous, kneadable consistency, particularly in case of a bone cement, air bubbles which may be entrapped during the mixing process can be removed only by an extremely high vacuum, if at all. During such evacuation, however, volatile constituents are also removed from the paste, which results in an unpredictable alteration of the mixing ratio of the cement and a deterioration of its chemical and physical properties.